I would suggest driving the motor with 72% of rated current, or about 310mA. That should yield the same power dissipation as would be expected from unipolar drive at 440mA. When the motor isn't moving, the voltage would have to be about 31 volts. I wouldn't expect insulation problems at 31 volts, or even 50, but I don't know how far one could safely go. Perhaps measure the AC voltage when hand-spinning the motor quickly to see if there seems to be a limit.
For switching purposes you should be using the saturation voltage guarantees rather than the DC current gain. For example, the 2N4401 is guaranteed at Ic/Ib = 10, so a base current of ~26mA will typically result in 220mV or so drop at 260mA (eyeballing the below plot).
The reason they say "pulsed" is that they are not accounting for the self-heating of the transistor. 26mA is quite a bit of base current, and the 220mV (typical!) drop will result in about 10 or 20 degrees C heating for the transistor (depending on the package).
If this is a hobbyist thing, you can probably get away with forced \$\beta\$ = 20-30 or so, so a base current of more like 9mA rather than the \$\beta\$ = 10 shown in the plot. I usually use \$\beta\$ = 20 at Ic <= 100mA, which might be a bit on the high side for a microcontroller to supply directly in your case (13mA of base current).
Edit: A "forced beta" of, say, 20 can be achieved as follows. Suppose that the collector current is limited the load resistance to say 200mA. No matter how well the transistor turns on it will never exceed about 200mA (even a dead short). You use a base resistor so the base current is 20mA. The ratio of Ic/Ib is now 20, and we can call that a "forced beta". The gain of the transistor at 200mA would be more like 150, but that's only when Vce is high enough that the gain determines the collector current. The datasheet specifies it at Vce = 5V, which would be a disaster for a switching application.
You can "force" the ratio of Ic/Ib to be as low as you like, but the upper limit is constrained by the transistor characteristics. When you're designing, ideally you'd like to eliminate the exact transistor characteristics, so that the circuit functions provided the transistor meets the minimum/maximum guarantees. The gain should never determine the collector current in a saturated switching application.
A small MOSFET with logic-level gate might be a better choice when you get much above about 100mA. As you can see, the performance starts to go a bit sour above approximately that current, even on the 2N4401.
Best Answer
In milling machines constant current is used to drive stepper motors.
You want the maximum torque of your stepper motors when using them (in the cnc machine). To achieve this, you would thus use constant current.
However, you do also want a reasonable speed of your motors. Therefore the supply that delivers the current, should be capable of also giving high enough voltages. If your constant current source is supplied with e.g. a 5V voltage, the maximum voltage of you stepper motors can be 5V, which will not give them a lot of speed. When you use for example a 30V supply, your constant current circuit will limit the curent and therefore protect your motor from breaking and the voltage will deliver the speed you need.
The voltage rating is just given as a maximum for when te motor is not turning.